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We all called it tar, assuming it was connected to high tar cigarettes. But maybe it's something else. Wiki says it's 'glandular trichomes'. Hairs under the leaf secrete black sticky stuff which functions as an insect trap. After a day of having your arms brushed by the hairs, it looks like a road surface has been laid on your arm. But maybe the tar which medics talk about is the residue of burnt nicotine.
When I picked it we got a layer of tar on the backs of our hands and our forearms. It was so thick that it was crumbling at the knuckles. When we passed the plants the hairs on our forearms would get pulled out, so we shaved them.